Gas turbine engine components employ coatings over base material in various applications to provide protection to the underlying structural base material. The purposes for applying the coatings are varied, and may include one or more purposes depending upon the application. For example, turbine airfoils used in the hot section or turbine section of a gas turbine engine include coatings to provide heat resistance and improved thermal capabilities. These airfoils also may be used in harsh environments, thereby additionally requiring coatings having resistance to corrosion or oxidation. Such coatings are referred to as environmental coatings.
Other applications may require still other coatings. For example, the shrouds surrounding the rotating airfoils, also referred to as blades, form a tunnel through which the hot gases pass. In addition to being able to withstand the hot, corrosive gases of combustion, these shrouds must also be abradable, as the rotating airfoils (blades) expand in a radial direction and contact the shrouds. It is desirable that, as contact is made between the rotating blades and the stationary shrouds that material be abraded from the shroud without affecting the structural integrity of the shrouds.
Of course, other sections of a gas turbine engine may require still other coatings. For example, in an aircraft engine, some components exposed to sand and rain may require erosion resistance. The specialized coatings in turbine engines are myriad. The coatings also may be applied to very large surface areas, such as shrouds, or to very small surface area, such as the tip regions of first stage turbine blades. The coatings also may be applied to a variety of substrate materials, such as for example superalloy materials, including nickel-based superalloys, cobalt-based superalloys, iron-based superalloys and combinations thereof, titanium and its alloys, and composites such as CMC's.
Although the structures to which the coatings are applied may vary, a few time-tested techniques have been utilized for their application. The techniques include a variety of modifications that solve particular problems. However the techniques generally include physical vapor deposition techniques (PVD), chemical vapor deposition techniques, thermal spray techniques, pack cementation techniques laser deposition techniques and plating techniques. A large number of patents have issued dealing with variations of the above-mentioned techniques, and many volumes could be filled discussing the differences distinguishing these variations. These techniques, including the multiple variations, typically produce high quality coatings, as required for demanding applications such as aircraft gas turbine techniques. However, the various techniques used for these applications have differing drawbacks. Some of the above-mentioned techniques, such as PVD, deposit the coating material by a slow, expensive process. Other techniques utilize solvents or release organic effluents, many of which are undesirable. Others, such as laser processing, require very high energy sources and expensive equipment. Still other processes leave undesirable by-products, such as heavy metals, for example chromium, which must be disposed of as hazardous waste. In general, two primary technologies have evolved in the coating industry: liquid coating technology, which may also be referred to as wet coating technology and powder coating technology, which may be referred to as dry coating technology.
Examples of the liquid coating technology include organic solvent type coatings and aqueous emulsion type coatings. Organic solvent type coatings, which are obtained by dissolving main components, such as resins, in an organic solvent and adding thereto auxiliary components, such as coloring agents, have been used widely in various coating applications. However, problems have been encountered with the use of these coatings, including fire hazards, adverse effects on safety/hygiene and environmental pollution. Accordingly, increased attention is being directed to coatings that vaporize no organic solvent, particularly aqueous emulsion type coatings and powder coatings.
Aqueous emulsion type coatings, however, also have certain shortcomings. For example, resin particles and a pigment are typically dispersed stably in an aqueous medium and thus a hydrophilic substance, such as an emulsifier, is employed during the production process. Additionally, the resultant film is often inferior in properties, such as alkali resistance and water resistance. Moreover, the film frequently has low adhesivity to the material being coated. It also takes a significant amount of time to obtain a dried film, as compared to that of an organic solvent type coating, and if it is necessary to complete the film drying in a short amount of time, then special equipment is required at higher costs.
In contrast, powder coatings, which contain no organic solvent, have various advantages. For example, powder coatings typically have very low volatile organic content and release very little volatile material to the environment when cured. Powder coatings are also free from flammable solvents, adverse effects on safety/hygiene and environmental pollution. Further advantages include the ability to be stored in an ordinary storehouse; the amount of ventilation air in a spray booth can be minimized and the air can be recirculated, resulting in high energy efficiency; and the coating film obtained has no foams generated by the vaporization of solvent during film drying. Other advantages of powder coatings include use without the necessity of adjusting viscosity, solid content, etc.; the coatings can be easily recovered without staining the operation site and producing any waste; and powder that does not adhere to a surface can be recycled. Furthermore, powder coatings can be applied by automated coating procedures and, in view of the total cost including cost of materials, pretreatment cost, cost of coating operation, equipment cost, etc., these coatings are very economical as compared to organic solvent type coatings and aqueous type coatings.
Powder coatings generally comprise a solid-film forming resin, often with one or more pigments. Thermosetting powder coating compositions and their method of preparation are described in U.S. Pat. No. 6,649,267 to Agawa et al. Similarly, U.S. Pat. No. 6,531,524 to Ring, et al. describes powder coating compositions. Although powder coatings may be thermoplastic-based, they are typically based on thermosetting materials. Themoplastic based coatings melt and flow onto the substrate during increases in temperature, but do not undergo a chemical reaction. Themoplastic based coatings are typically applied to a greater thickness than that of thermosetting coatings.
In contrast, thermosetting powder coatings melt upon increase in temperature and undergo a chemical reaction to polymerize through cross-linking mechanisms into a resistant resultant film. These thermosetting coatings do not remelt once the chemical reaction has occurred.
In general, powder coating technology is an advanced method of applying decorative and protective finishes to products to enhance features, such as color and scratch resistance. Typically, the powder coating is applied by a spray technique wherein the powder constituents are sprayed onto an article and then heated to fuse the powder onto the article. The powder particles are attracted to the article by an electrical charge. Industries that have benefited from powder coating technology include the appliance and architecture industries.
However, to the inventors' knowledge, powder coating technology has not been employed to coat gas turbine engine components in the aerospace industry. In particular, gas turbine engines operate at increasingly high temperatures due to the increased desire for further efficiency. Accordingly, the gas turbine engine components must be able to withstand the increased temperatures and thus coatings are often employed over the components to provide further protection. In particular, numerous coatings are used in gas turbine engine systems for purposes of: heat/thermal control, sand/rain erosion resistance, wear resistance, corrosion resistance/sacrificial coatings, and many others. A number of these coatings use solvents, which may be harmful or toxic. Some coatings also include constituents that allow them to work for special applications, but are toxic (e.g. chromium) or release organic effluents during processing. Additionally, the coatings must often operate at temperatures anywhere from subambient to extremely hot (e.g. in excess of 2000° F./1093° C.).
Thermal spray processes, including detonation gun deposition, plasma spray, electric wire arc spray, flame spray and high velocity oxy-fuel, have been extensively used in the gas turbine engine industry to deposit coatings on various engine components. In most of these thermal spray processes, materials such as ceramic, polymeric or metallic materials in wire, powder or other forms are heated to at or above its melting point. Droplets of the melted material are directed against the surface of a substrate to be coated via a gas stream and adhere and flow onto the component where a buildup of coating results. However, these processes are often complicated and require extensive equipment and set up procedures. Moreover, thermal spray processes may also be characterized similar to the liquid coating technology, shortcomings of which have been described above in detail.
Accordingly, there exists a need for a new method of coating gas turbine engine components. What is needed is a process that can deposit a variety of coatings on aircraft engine parts in an economical, fast, energy efficient process that has minimal environmental impact. One method that has heretofore not been used in gas turbine components is powder coating based on electrostatic deposition of powdered materials. While this method has been used for a variety of commercial products such as home appliances, basketball poles, lawn furniture, gas grills and certain automotive applications, the methods have not heretofore been extended for demanding applications such as gas turbine components, including aircraft engine applications. It is likely that such methods have not found their way into this art because they lack a reputation for durability in such demanding applications. The coating should be a durable coating for gas turbine engines that is quick, cost effective and environmentally friendly, and which can be readily adapted for application to both large and small surface areas.